In recent years, energy-saving hydraulic oils have been developed as one of responses to global warming. There are some conventional energy-saving hydraulic oils allowing achieving the reduction of energy consumption of apparatuses at starting, for example, by decreasing their low-temperature viscosity.
There are also developed energy-saving hydraulic oils whose viscosity change is made small by blending a viscosity index improver to thereby reduce energy consumption in the steady-state operation after the fluid temperature is raised. In the energy-saving hydraulic oils, the fluid leakage (internal leakage) from construction machines' characteristic various hydraulic apparatus interiors is prevented by making small the viscosity change (making the viscosity index high) of the hydraulic oils, and the reduction of the energy consumption is achieved (for example, see Patent Literatures 1 to 3).